Remote subscriber interaction system

ABSTRACT

A central control unit (scanner) is connected across a set of subscriber loops. At each subscriber&#39;s premises, an individual subscriber terminal unit (STU) is connected across that particular subscriber&#39;s loop. To each STU are connected the various instrumentalities which are to be monitored by the system, e.g. fire alarms, burglar alarms, etc. FSK modulated signals in the upper part of the audible range are transmitted from the scanner to the STUs at appropriate times. The STUs reply by means of similar signals to indicate the status (e.g. alarm, or non-alarm) of the instrumentalities at the respective subscriber premises. This transmission and retransmission is interrupted when the subscriber&#39;s telephone is off-hook. In addition, a signal below the audible range (low tone) is produced at the STU, and transmitted to the scanner over the telephone loop if, but only if, all the instrumentalities at the particular subscriber premises are in a given state (e.g. non-alarm). If not, low tone is stopped. Such stoppage is sensed by the scanner, and causes immediate transmission of the FSK signals from the scanner to the corresponding STU, even though the associated telephone is off-hook at the time. The reply to such transmission provides information about the reason for the stoppage, i.e. what is the source of the alarm.

This invention relates to systems for utilizing telephone networks toprovide additional functions, beyond those of traditional telephoneservice involving conversations between subscribers. More particularly,the invention relates to such systems in which the network isadditionally used to monitor the conditions of one or moreinstrumentalities, located at the subscriber's premises.

The instrumentalities in question may be of various types. One type is adevice for providing an electrical alarm signal when an abnormalcondition is detected at the subscriber's premises. Such an abnormalcondition may be a fire, an unauthorized entry, a malfunction of somemachinery under surveillance, etc. Another type may be a device whichprovides an electrical signal indicative of the state of some equipmentat the subscriber's premises, even though there is no abnormality. Thiscould be utility meter readings, meteorological measurements,operational data of process equipment, etc. Still other types can beenvisioned.

The present invention is by no means the first to deal with systems ofthis kind. Indeed, numerous such systems have previously been proposed.However, so far as the present inventors are informed, all suchpreviously proposed systems had characteristics which detracted fromtheir usefulness, or their desirability, in one way or another.

For example, some prior systems proposals involved the use of auxiliarysignals which were audible to the subscriber when using the telephonefor conventional conversational purposes. This is not only disturbing tothe subscriber, but also contravenes the technical restrictions imposedby some telephone companies, or governmental regulatory agencies.

Other such prior systems proposals involved the use of auxiliary signalswhich were above the range of audible frequencies. However, this createdelectrical filtering requirements within the system itself, and possiblyalso within the conventional telephone network, which were difficult toimplement, and also costly, to the point of impairing the economicacceptability of the system.

Still other system proposals, in an effort to overcome drawbacks such asnoted above, simply sacrificed some desirable system functions, such asreliable surveillance of the equipment at the subscriber's premises, orthe like.

Accordingly, it is a principal object of the present invention toprovide a system which cooperates with a conventional telephone networkto provide additional functions, but in an improved manner as comparedwith previously proposed systems intended for the same general purpose.

It is another object of the present invention to provide such a systemwhich lacks one or more of the shortcomings of previously proposedsystems.

It is another object to provide such a system which does not useauxiliary signals that are audible to the subscriber during normaltelephone usages.

It is another object to provide a system which does not rely on signalsabove the audible frequency range.

It is another object to provide a system which does not require specialelectrical filtering within the conventional telephone network.

It is another object to provide a system which provides a high degree ofreliable surveillance of the equipment at the subscriber's premises.

These and other objects which will appear are accomplished in accordancewith the present invention as follows.

At a point at which a group of subscriber loops to be served by thesystem is conveniently accessible, a central control unit (hereinaftercalled "scanner") is connected across all these subscriber loops. Ateach subscriber's premises, an individual electronic unit (hereaftercalled "subscriber terminal unit" or "STU") is connected across thatparticular subscriber's loop.

To each STU, there are connected the various instrumentalities which areto be monitored by the system, so that their output signals areavailable in electrical form within the STU.

By means of suitable signals within the audible range, the scannerinterrogates the STUs at appropriate times. The STUs reply by means ofsimilar signals to indicate the status of the instrumentalities at therespective subscriber premises.

This normal pattern of interrogation and reply is limited to periodsduring which the subscriber's telephone is on-hook, i.e. when thesubscriber is not using the telephone for conventional purposes, andceases when the subscriber picks up the telephone (i.e. creates anoff-hook condition).

A special signal at a frequency well below the audible range is alsoproduced at the STU, and is transmitted to the scanner over thetelephone loop, which of course also carries the other conventionaltelephone signals: voice, dial tone, ringing, busy tone, etc. Thisspecial signal (hereafter called "low tone") is preferably presentcontinuously, during both on- and off-hook conditions, but only if allthe instrumentalities at the particular subscriber premises are withintheir predetermined normal, or safe, range of states.

If any one or more of these instrumentalities depart from that range,i.e. assumes a state of danger or alarm, production of low tone withinthe STU is discontinued. Such discontinuance of low tone is sensed bythe scanner, and causes immediate interrogation of the particular STUwhich is the source of this phenomenon, even though the associatedtelephone is off-hook, so that this STU would normally not beinterrogated (or reply) at this time. The reply to such interrogationwhich is then produced by the STU provides information about the reasonfor the observed phenomenon, i.e. what is the source of the danger oralarm.

As previously noted, interrogation and reply take place within theaudible frequency range, and are therefore superposed on any other useof the telephone during the off-hook condition, e.g. upon subscriberconversations. However, this occurs only in response to a danger oralarm indication. Therefore it is not considered objectionable, nor isit prohibited by telephone regulations. Indeed, it actually has thedesirable aspect of simultaneously alerting the telephone user to thesituation.

In the absence of a phenomenon such as described above, the scannernormally interrogates the individual STUs in a predetermined, cyclicalsequence. However, other deviations from that sequence are preferablyalso provided. For example, when a particular STU is first activated aspart of the system, it is preferably interrogated immediately, eventhough this may be out of its normal sequence, to determine the statusof its associated instrumentalities without delay. Also, whenever asubscriber hangs up, the STU at that location is preferably immediatelyinterrogated, even though low tone had not indicated a danger or alarmcondition.

For further details, reference is made to the discussion which follows,in light of the accompanying drawings wherein:

FIG. 1 is an over-all system block diagram of an embodiment of theinvention;

FIG. 2 is a block diagram of a scanner forming part of the embodiment ofFIG. 1; and

FIG. 3 is a block diagram of a subscriber terminal unit (or STU) formingpart of the embodiment of FIG. 1.

The same reference numerals designate similar elements in the differentfigures.

Referring to FIG. 1, this shows a telephone network 10 which, in itself,is entirely conventional in all respects. Indeed it is one of thefeatures of the present invention that it can be practiced using aconventional telephone network, without interfering with its operationin the slightest.

This network 10, which is shown in highly simplified form in FIG. 1,includes a plurality of subscriber telephone handsets, respectivelydesignated by reference numerals 11, 12 and 13. Each of these handsetsis connected to its own local telephone line, or loop. These arerespectively designated by reference numerals 14, 15 and 16. These locallines, in turn, all lead to a switching system 17, normally located atthe telephone central office. All of these elements of the system maytake essentially any conventional form. The subscriber handsets 11, 12,13 may be of the dial or pushbutton variety. The central office switch17 may be of the rotary, or cross-bar, or even fully electronic variety.The local lines 14, 15, 16 may also be of any conventional variety,including repeaters, equalizers, etc., as appropriate, provided onlythese are capable of passing the low tone signals.

In accordance with the present invention the system of FIG. 1 furtherincludes a scanner 18. This scanner is preferably physically located atsome point at which it can conveniently be connected to the variousindividual subscriber lines. This may be at the central office, wherethe switching equipment 17 is also located. In FIG. 1, connections 19,20 and 21 are shown between the scanner 18 and the respective lines 14,15 and 16. Further in accordance with the present invention, at eachindividual subscriber location, there is provided a separate subscriberterminal unit, abbreviated as STU, respectively designated by referencenumerals 22, 23 and 24.

Attention is invited to the fact that equipment for only threesubscribers is shown in FIG. 1. However, it should be understood thatthis is only for simplicity of illustration. The invention is intendedto be used with larger numbers of subscribers, in which case thehandsets, the connections and the STUs would all be correspondinglyincreased in numbers. This is symbolically indicated in FIG. 1 by widespaces bridged by broken lines, between all the foregoing elements ofthe overall system.

Turning now to FIG. 2, which shows the "innards" of the scanner 18,these include first, second and third multiplexers 25, 26 and 27. Theseare all time-division multiplexers which, in conventional manner, selectthe signals on different ones of connections 19, 20 and 21 at differenttimes, and transmit the so-selected signals to their respective outputs.Moreover, multiplexer 25 is a two-way device, which is also capable ofdistributing selectively, at different times, to the differentconnections 19, 20 and 21, signals supplied to the other "end" of thismultiplexer 25. Thus, either end of multiplexer 25 can serve as inputand output, as appropriate. The multiplexer 25 is connected to atransmit-receive switch 28 (also abbreviated as T/R switch). The portionof T/R switch 28 which operates in the receive mode in turn suppliessignals from multiplexer 25 to a demodulator 29 for frequency-shiftkeyed (abbreviated as FSK) signals. The portion of T/R switch 28 whichoperates in the transmit mode receives its input signal (to betransmitted) from a modulator 30 for FSK signals.

In turn, demodulator 29 supplies its output signals to system controller31, and modulator 30 receives its input signals from the same controller31.

As for multiplexers 26 and 27, these supply their respective outputsignals to different filters 32, 33. In turn, filter 32 supplies itsoutput signals to hook detector 34, while filter 33 supplies its outputsignals to low tone detector 35. Both of these detectors ultimatelysupply their output signals to the same system controller 31 which waspreviously mentioned.

Because of the interlocking relationship between the operations of thevarious elements of scanner 10 which are shown in FIG. 2, and elementsof each STU 22, 23 and 24 (see FIG. 1), it is believed necessary todescribe the latter in further detail before dealing more fully with theoperational characteristics of either. To that end, reference is nowmade to FIG. 3, which shows the "innards" of STU 22. It will beunderstood that all STUs may be substantially identical in constructionso that the description of STU 22 presented herein would apply equallyto STUs 23 and 24, and to any additional STUs symbolized by the dashedlines between STU 23 and STU 24 in FIG. 1.

This STU 22 (FIG. 3) is connected to line 14, which also connectssubscriber handset 11 to the network switch 17 and to scanner 18.

Signals received from line 14 are supplied within STU 22 to a filter 36,the output signal from that filter being supplied to a demodulator 37for frequency-shift keyed signals.

Supplied to line 14 are signals from a modulator 38 for frequency-shiftkeyed signals.

Also supplied to line 14 are signals from a low tone signal generator39.

In turn, the output signals from demodulator 37 are supplied to signalprocessing circuitry 40, and the input signals to modulator 38 and togenerator 39 are derived from that same signal processing circuitry 40.

Signal sources 41, 42 and 43 in FIG. 3 represent electrical signaloutputs from corresponding instrumentalities (not shown) which arelocated at the same premises as the STU 22 and handset 11, and which areintended to be under the surveillance of the system which embodies thepresent invention.

For example, signal source 41 may be a relay which closes and therebyestablishes an electrical signal path whenever a fire alarm device atthe subscriber's premises senses an alarm condition. By the same token,signal source 42 may be a relay which operates similarly when a burglaralarm is activated, and signal source 43 may function similarly in caseof interruptions in a temperature control system.

Proceeding now to the operational characteristics of the elementsdescribed above, it is desired to point out first that there is whatmight be called a "closed-loop" relationship between the scanner 18 andeach STU 22, 23 and 24. That is, the scanner produces signals which aretransmitted to the STU, which responds thereto to transmit signals backto the scanner, and so on in back-and-forth, or closed-looprelationship.

For description here, let us therefore initially break into this closedloop at that output from system controller 31 which is connected to FSKmodulator 30 (FIG. 2). This system controller is constructed so as torecurrently produce at that output FSK modulating signals in apredetermined pattern. This pattern is such that the STUs 22, 23, 24forming part of the overall system respond to it to provide, in turn,certain output signals as described hereafter. The FSK modulator 30responds by producing an FSK-modulated equivalent of the signals fromcontroller 31. The two carrier frequencies used are preferably in thevicinity of 2,700 and 2900 Hz, i.e. near the upper end of the band offrequencies passed by a typical local telephone line.

These FSK-modulated signals from modulator 30 then pass through T/Rswitch 28 to multiplexer 25. There, control signals are also appliedfrom system controller 31, in such time coordination with the signalsfrom FSK modulator 30, that the latter are channeled to one or anotherof connections 19, 20 and 21, depending upon which one of STUs 22, 23 or24 is the intended destination of these signals from FSK modulator 30 atthat time.

Assuming that the intended destination for a particular signal train isSTU 2, then multiplexer 25 supplies that train to connection 19, whenceit reaches STU 22 via telephone line 14. Note that the same signal trainalso reaches handset 11 via line 14. More will be said about this later.

Upon reaching STU 22, and referring now to FIG. 3, the signal traindescribed above is demodulated in FSK demodulator 37, after passagethrough filter 36, which is preferably a band-pass filter constructed toselectively transmit the band of frequencies occupied by the two FSKcarrier frequencies (e.g. the 2,700 to 2,900 Hz band). At the output ofdemodulator 37, there are therefore reconstructed and modulating signalsoriginally provided by controller 31 (FIG. 2). These reconstructedsignals are supplied to signal processor 40 within STU 22.

This processor 40 is so constructed as to respond to the above-mentionedsignals to generate a train of modulating signals which are in turnsupplied to FSK modulator 38, where they produce a corresponding trainof FSK-modulated signals, preferably at the same carrier frequencies asthose received from scanner 18. This train of FSK-modulated signalsreturns to that scanner via line 14 and connection 19. In passing, it isnoted that this response signal train of course also reaches handset 11.

At the scanner, the multiplexer 25 is activated by controller 31 in timecoordination with the expected arrival of this responsive signal trainfrom STU 22 so as to pass signals on connection 19 to the FSKdemodulator 29, via the receive portion of T/R switch 28. Demodulator 29recovers the signals which had been used to modulate the FSK signals atthe STU 22 and supplies same to system controller 31.

Reverting now to STU 22 (FIG. 3), the modulating signals produced bysignal processor 40 provide information about the state of the signalsources 41, 42 and 43. For example, if at STU 22 there are present norelay closures denoting alarm states of the various instrumentalitiesunder surveillance, then one predetermined pattern of modulating signalsis produed by processor 40. If one or more relay closures are present,then correspondingly different patterns are produced, and the(demodulated) signals which ultimately reach the controller 31 from STU22 are also different. The system controller 31 responds to theparticular pattern of signals received in a particular way. For example,if a pattern of signals is received which indicates that there is analarm state at signal source 41 of STU 22 (FIG. 3), the systemcontroller 31 provides a corresponding alarm signal to any desiredindicator. For example, this may be an indicator at the fire departmentwhich is in charge of the subscriber's premises at which the STU 22 islocated. Such uses of the signal pattern received from each STU areconventional, in themselves, and may take any of numerous forms, withoutaffecting those aspects of the overall system which constitute thepresent invention. The means for accomplishing these uses are thereforenot further described or illustrated herein.

Attention is now again directed to the fact, previously noted, that theFSK signals passing back and forth between scanner 18 and any given STUalso reacn the telephone handset with which that STU is associated. Thisis not a problem, so long as the telephone is on-hook, since these FSKsignals remain unnoticed by the subscriber during such on-hookconditions. Not so during off-hook conditions. The FSK signals arewithin the audible frequency range. They therefore are heard by thesubscriber when trying to use the telephone for normal communicationpurposes. This is quite disturbing, especially since these FSK signalsare intentionally caused to recur at frequent intervals, in order toprovide indications of the state of the instrumentalities undersurveillance which the relatively up-to-date. Moreover, even if theaudible effect are these FSK signals were considered acceptable by thesubscriber, they would still in many instances contravene telephonecompany and/or governmental regulations, which prohibit the introductionof such extraneous audible signals into the network during off-hookconditions.

In accordance with the present invention, this matter is dealt with asfollows.

The generator 39 which is provided at each STU (FIG. 3) is constructedso as to produce a signal at a frequency substantially below the audiblefrequency range. For example, this generator 39 may be capable ofproducing a signal of approximately 25 Hz frequency. It is because ofits operation at such a low frequency that generator 39 is referred toas a "low tone" generator.

The generator 39 does not operate continuously. Rather it is controlledby signal processor 40 so as to be in one or the other of two states. Solong as the signal sources 41, 42 and 43 are all in their non-alarmstate, the signal processor 40 causes the low tone generator 39 toproduce its low frequency output signal, which thereupon reaches thescanner 18 via telephone line 14 and connection 19. In contrast,whenever any one or more of signal sources 41, 42 or 43 is in an alarmstate, then signal processor 40 causes the low tone generator 39 to stopproducing its low frequency output signal. This low frequency signaltherefore also ceases to reach scanner 18. As previously indicated, eachSTU is equipped in substantially similar manner as the STU 22 which isshown specifically in FIG. 3, and operates in substantially the sameway.

At that scanner 18, multiplexers 26 and 27 are energized by systemcontroller 31 so as to sequentially sample the signals appearing onconnections 19, 20 and 21, respectively.

From multiplexer 26, a signal representing, in recurrent sequence, thehook condition of the different handsets 11, 12, and 13 is derived viafilter 32 and hook detector 34, and supplied to system controller 31.

From multiplexer 27, a signal representing, also in recurrent sequence,the presence or absence of the low frequency signals from the differentlow tone generators of STUs 23, 23 and 24 is derived via filter 33, andsupplied to system controller 31.

Observe now what happens during each of four possible situationsprevailing at any given STU, e.g. STU 22 of FIG. 3.

One possible situation is that the telephone handset 11 associated withSTU 22 is on-hook, and no alarm condition prevails at STU 22. Theon-hook condition is sensed at scanner 18 and the output from FSKmodulator 30 is periodically supplied to that STU 22, which respondswith a non-alarm output from its FSK modulator 38. This is detected byFSK demodulator 29 and system controller 31 takes no alarm action, butsimply continues the periodic sampling of the connection to this STUboth for continued on-hook signal and for continued non-alarm FSKsignals.

A second possible situation is that the telephone handset 11 is stillon-hook, but an alarm condition prevails at STU 22. The on-hookcondition is still sensed at scanner 18 and the output from FSKmodulator 30 is still supplied periodically to that STU 22. However, theSTU now responds with an alarm signal from its FSK modulator 38. This isdetected by FSK demodulator 29, and system controller 31 takes theappropriate alarm action, as previously described.

A third situation is that the telephone handset 11 is off-hook, and thatthere is no alarm condition at STU 22. This off-hook condition is nowsensed at scanner 18, which thereupon does not supply output signalsfrom FSK modulator 30 to STU 22 for the duration of this thirdsituation, provided the scanner 18 also senses a low tone signalreceived from the same STU. It will be recalled that a low tone fromgenerator 39 is produced only when no alarm condition prevails.

The fourth situation is that the telephone handset 11 is again off-hook,but that there is at least one alarm condition at STU 22. The off-hookcondition at STU 22 is again sensed at scanner 18, but so is the absenceof the low tone signal from that same STU. Under these circumstances,the scanner 18 does supply output signals from FSK modulator 30 to STU22, of the same type as it would have supplied had the handset 11 beenon-hook, rather than off-hook. This will, of course, cause a reply fromSTU 22, in the form of FSK modulated signals from its modulator 38,which will in turn be detected at scanner 18, and used to produce thedesired alarm response.

It will now be recognized that, so long as there is no alarm conditionat the STU, there will be no signal in the audible range from theoperation of the present invention while the associated handset is inuse by the subscriber. This satisfies the subscriber's desires and alsocomplies with regulations. On the other hand, when there is an alarmcondition, there will be such audible signals, even when the handset isin use, in the form of the FSK modulated signals travelling back andforth between scanner location and subscriber premises. However, duringalarm conditions, the presence of such audible signals is not onlypermitted by regulations, but is also affirmatively desirable from thesubscriber's standpoint, since it provides prompt warning to thesubscriber that an alarm condition has arisen.

It will also be understood that, by means of the multiplexers 25, 26 and27 in scanner 18 (FIG. 2), the same treatment which is accorded to STU22 is also accorded to STUs 23 and 24, in recurring sequence. That is,first the scanner 18 is connected so that it interacts with STU 22 inthe manner described above, then it is connected with STU 23 for theequivalent purpose, then with STU 24, and so forth in recurringsequence.

Moreover, if there are more than three STUs--which will normally be thecase in any practical application--these are all treated in recurrentsequence in the manner described above for the single STU 22.

A number of additional observations about the present invention arebelieved to be in order.

A particulary "happy" combination of interacting features is believed tobe provided by relying primarily upon the high-frequency end of theaudible range for on-hook alarm reporting purposes, while relying upon asub-audible signal for initial off-hook alarm reporting purposes.

Use of the high-frequency end makes a relatively high data ratepossible, which means that relatively detailed information about theinstrumentalities under surveillance can be obtained, and also that thiscan be repeated frequently, and relatively many STUs can be accommodatedby the system. Use of the sub-audio end makes it possible to speakwithout interference during the (presumably overwhelming) majority ofthe times when no alarm is present. Yet, by shifting from sub-audio tohigh frequency, the desired detailed information can be obtained whenneeded, i.e. when an alarm condition arises, even during off-hookperiods.

It will also be noted that low tone provides redundancy for the alarmfunction of the system during on-hook conditions. During that condition,low tone continues to be transmitted from each STU to the scanner, solong as no alarm is present at the STU. Any low tone interruption duringon-hook conditions is also sensed by the scanner, and provides whatmight be called a "coarse" back-up alarm indication for the moredetailed one which is normally provided by the FSK-modulated reply fromthe STU.

Moreover, the alarm indication during off-hook periods is fail-safe,since it is the disappearance of the low tone signal which indicates analarm condition. Thus, failure of equipment in the STU will cause apseudo-alarm indication conducive to prompt correction.

In a preferred embodiment of the invention, there is one other occasion,in addition to that described above, when a particular STU has the FSKmodulated signal train from scanner 18 transmitted to it, even though itis not that STUs regular turn, in recurrent sequence.

That occassion is the return from off-hook to on-hook status of itsassociated telephone handset.

It will be recalled that the hook status is detected for each STU at thescanner 18. When a change from off- to on-hook status is thus detected,the STU at that subscriber's premises is preferably made the next one toreceive the FSK-modulated signal from the scanner, and therefore alsothe next one to provide its FSK modulated reply. The reason for this isthat only non-detailed information about the status of theinstrumentalities at that location had been available during thepreceding off-hook period, and it is deemed desirable to up-date thiswithout delay once the off-hook condition no longer prevails.

Another highly desirable feature of the invention is that the individualcomponents of the overall system can take any one of a number of knownforms.

Thus, all of the elements of the scanner 18 (FIG. 2) may be of knownform, and so may all the elements of the STU 22 (FIG. 3).

Considering first the scanner 18, it will be recognized that themultiplexers 25, 26 and 27 may be of known construction. The filters 32and 33 may be of known form, suitable for selecting the particularfrequencies desired to be passed on to the hook detector 34 and low tonedetector 35, respectively. These detectors, themselves, may also be ofknown form, the hook detector 34 being a conventional part of telephonesystems, and the low tone detector 35 being a detector of the presenceor absence of the 25 Hz low tone signal. The T/R switch 28 may be ofknown forms, and the system controller 31 may include any knowncircuitry capable of providing the straightforward sensing andresponsive control functions which have been described. Thus, systemcontroller 31 may comprise a conventional source of timing signals, fromwhich are derived the sequential operating signals for the multiplexers25, 26 and 27. Also driven by signals derived from the timing signalsmay be sources, such as shift registers, of the trains of modulatingsignals for FSK modulator 30. AND gate circuitry may be used to detectthe simultaneous presence of off-hook detected signals and low tonedetected signals. Such detection is used by conventional circuitry toinhibit signals to the FSK modulator 30. Conversely, the non-detectionof these simultaneous signals causes the transmission of such signals tothe FSK modulator.

Considering next the STU 22, the FSK demodulator and modulator 37 and 38may again be of any known form, as also the filter 36 which precedes thedemodulator 37. The low tone generator may also be of any known formcapable of producing the desired 25 Hz signals when energized. Thesignal sources may be simple relay circuits operated by the respectivealarms, and the signal processor may be any known form of digital logic,or of microprocessor circuitry programmed in conventional manner toelicit the appropriate timed signal responses, as previously described.

The telephone system itself is completely unaffected by the invention.No added filtering is required in the telephone system; in particularthere is no leakage of low tone signals from one side of the networkswitch 17 to the other because the network switch contains adequatefiltering for other reasons, namely to isolate different levels of DC onopposite sides of the switch.

When the system embodying the present invention is used, not to signalan alarm, as such, but rather other states of equipment at a telephonesubscriber's premises, such as meter readings, meteorologicalmeasurements, etc., then low tone may be used to indicate that all themeasuring instruments are continuing to work, even while the telephonehandset is off-hook. During on-hook periods, the signals from any givenSTU can then supply the more detailed data desired concerning theequipment.

It will be understood that various modifications will occur to thoseskilled in the art without departing from the inventive concept, whosescope it is desired to define only by the appended claims.

We claim:
 1. A system for providing communication between a centrallocation and instrumentalities other than a conventional telephonelocated at the premises of telephone network subscribers,comprising:means for selectively transmitting signals from the centrallocation to the telephones at different ones of said subscriberpremises; means located at said premises and connected to saidtelephones and responsive to said transmitted signals to retransmit tothe central location from the respective subscriber premises signals inthe audible telephone frequency range and of characteristics indicativeof the state of said instrumentalities; means at the central locationfor sensing the off-hook state of the conventional telephone at a givensubscriber's premises and responding thereto to inhibit the transmittingof said signals from the central location to that same premises; meansfor transmitting to the central location a signal at a frequency belowthe audible range from each of the premises when the instrumentalitiesat the given subscriber premises are in one predetermined state; andmeans responsive to at least one instrumentality being in anotherpredetermined state to interrupt the transmission of saidbelow-audible-range signal.
 2. The system of claim 1 wherein thebelow-audible-range signal is transmitted from any given premises atleast during off-hook conditions of the telephone at the same premises.3. The system of claim 1 wherein the below-audible-range signal istransmitted from any given premises both during on-hook and off-hookconditions of the telephone at the same premises.
 4. The system of claim1 wherein means are provided for taking an alarm-responsive action atthe central location in response to interruption of thebelow-audible-range signal.
 5. The system of claim 1 wherein the signalstransmitted from the central location are FSK modulated signals in theupper portion of the audible range.
 6. The system of claim 5 wherein thesignals retransmitted from the subscriber premises are also FSKmodulated signals in the upper portion of the audible range.
 7. Thesystem of claim 6 wherein the FSK carrier frequencies are in thevicinity of 2700 and 2900 Hz.
 8. The system of claim 1 wherein thebelow-audible-range signal is at a frequency of approximately 25 Hz. 9.The system of claim 1 whereinindividual wire connections are provided atthe central location to each subscriber local loop; and multiplexingmeans are provided for applying the transmitted signals selectively todifferent ones of said individual connections in predetermined sequence,and for selectively receiving the signals from the same said individualconnections in the same said predetermined sequence.
 10. The system ofclaim 1 further comprisingmeans for sensing the discontinuance of thebelow-audible-range signal during continued sensing of the off-hookcondition at a selected subscriber premises, and means responsivethereto to transmit the signals from the central location to the sameselected subscriber premises.
 11. The system of claim 1 furthercomprisingmeans for sensing the discontinuance of the off-hook conditionat any selected subscriber telephone, and means responsive thereto totransmit the signals from the central location to the same selectedpremises.
 12. A system for providing communication between a centrallocation and instrumentalities other than a conventional telephonelocated at the premises of telephone network subscribers,comprising:means for selectively tramsmitting signals from the centrallocation to the telephones at different ones of said subscriberpremises; means located at said premises and connected to saidtelephones and responsive to said transmitted signals to retransmit tothe central location from the respective subscriber premises signals inthe audible telephone frequency range and of characteristics indicativeof the state of said instrumentalities; means at the central locationfor sensing the off-hook state of the conventional telephone at a givensubscriber's premises and responding thereto to inhibit the transmittingof said signals from the central location to that same premises; meansfor transmitting to the central location a signal at a frequency belowthe audible range from each of the premises when the instrumentalitiesat the given subscriber premises are in one predetermined state;individual wire connections at the central location to each subscriberlocal loop; multiplexing means for applying the transmitted signalsselectively to different ones of said individual connections inpredetermined sequence, and for selectively receiving the signals fromthe same said individual connections in the same said predeterminedsequence; means for sensing the discontinuance of thebelow-audible-range signal during continued sensing of the off-hookcondition at a selected subscriber premises; and means responsivethereto to transmit the signals from the central location to the sameselected subscriber premises.
 13. A system for providing communicationbetween a central location and instrumentalities other than aconventional telephone located at the premises of telephone networksubscribers, comprising:means for selectively transmitting signals fromthe central location to the telephone at different ones of saidsubscriber premises; means located at said premises and connected tosaid telephones and responsive to said transmitted signals to retransmitto the central location from the respective subscriber premises signalsin the audible telephone frequency range and of characteristicsindicative of the state of said instrumentalities; means at the centrallocation for sensing the off-hook state of the conventional telephone ata given subscriber's premises and responding thereto to inhibit thetransmitting of said signals from the central location to that samepremises; means for transmitting to the central location a signal at afrequency below the audible range from each of the premises when theinstrumentalities at the given subscriber premises are in onepredetermined state; individual wire connections at the central locationto each subscriber local loop; multiplexing means for applying thetransmitted signals selectively to different ones of said individualconnections in predetermined sequence, and for selectively receiving thesignals from the same said individual connections in the same saidpredetermined sequence; means for sensing the discontinuance of theoff-hook condition at any selected subscriber telephone; and meansresponsive thereto to transmit the signals from the central location tothe same selected premises.